Some 6 year old children have a very different energy level than is safe to be around the infants and toddlers. They are bigger, move faster, and have more energy to be active.
We did test out having 6 year-olds in when we first opened the Play Center, but found it to be boring for them and unsafe for the younger children.
There are other things 6 year-olds are able to do while you work out, just ask the front desk for options.